This invention relates generally to gas turbine engines and more particularly to methods for operating such engines during a shutdown period.
A gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure or gas generator turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. In a turboshaft engine, another turbine disposed downstream of the core (referred to as a low pressure, “work”, or “power” turbine) extracts energy from the primary flow to drive a shaft or other mechanical load. One common use is to couple the gas turbine engine to an external load such as a pump, compressor, or electrical generator.
For efficient operation, the turbomachinery in a gas turbine engine depends on maintaining small but definite radial clearances between the tips of the rotating blades and the stationary annular casing that surrounds them. The casing is generally more “thermally responsive” than the rotor, i.e. it generally expands or contracts at a greater rate than the rotor during a change in engine power output, and the associated temperature change. As a result the blade clearances tend to open or close during changes in engine power output. For this reason, gas turbine engines are generally shut down by gradually reducing the output power level, so that the radial clearances can stabilize.
However, operational reasons can require that the external load be removed and that the engine be shut down immediately, without being able to gradually reduce power. This is referred to as a “hot shutdown”. When a hot shutdown occurs, the engine components cool rapidly. In general the casing cools down faster than the rotor, causing the case to compress against the rotor blades and close the airflow clearances in the compressor. Also, once the engine stops rotating natural convection patterns cause the upper portions of the rotor to heat up and expand more than the lower portions. This causes bending or bowing of the rotor that further reduces radial clearances at specific locations. The combined effect of case shrinkage and rotor bowing cause the rotor to become “locked”, a condition in which the rotor and casing actually contact each other.
When the engine experiences a hot shutdown, the engine must be restarted or undergo a hot crank within a short time after the shutdown (for example about 10 minutes) in order to prevent the rotor from locking up. If the rotor locks up, the engine cannot be restarted until after the passage of a “lockout period”, in order to avoid rotor and casing damage. This period is undesirable for a number of reasons including the cost and physical inconvenience of not having the engine in service.
Accordingly, there is a need for a method of operating a gas turbine engine that minimizes or eliminates the lockout period after a hot shutdown.